[opendtv] Re: RV: Re: Charles Rhodes on unlicensed devices and white spaces
- From: "johnwillkie" <johnwillkie@xxxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 9 Aug 2007 14:28:07 -0700
Gee, Richard, and somehow you are confused about the difference between
predicted signal strength, and ACTUAL signal strength?
And, did you ever note that actual signal strength changes over time, yet
the antennaweb.com web site (useful ONLY for selecting antennae) doesn't ?
You need to spend at least a bit of time to understanding the issues. The
"White Spaces" change channel by channel. If this were intended to be a
rural service, the white areas would be immense.
But, it's intended as an urban service.
"Very close" isn't good enough: broadcasters are allocated their channels as
a primary service, which means that all other services must not cause
interference to the primary user, and must accept interference from the
primary user.
It's done every day with wireless mics. It's not something that you want to
trust to be "done right" by a 17-year old with raging hormones, a new little
box, and a desire to take down TV stations.
If they can't prevent interference to primary users - and they CAN'T - why
do we need to go on? They certainly won't be able to prevent interference
(or even detect it) from other similar users.
Also, some heart monitors in ICUs use these frequencies on a non-interfering
basis. Be kinda fun, don't you think, to take down a whole floor of heart
monitors; probably won't kill every patient attached to them, but it would
be "very exciting" in that ward for the duration.
Here's the litany.
First, they said they could detect all interference. The NAB probed
otherwise, so
Then, they said that interference wouldn't be caused. The NAB proved
otherwise, so
Then they said that they had a new fix to the interference issue, so the FCC
- fools they are - approved, contingent on their fix being proved.
Now, they're back to the drawing board.
The record is quite extensive, Richard. These devices are a non-starter,
unless there is a way that individual devices can be made tamper-proof AND
can detect and prevent all potential AND actual interference.
There is a reason why Part 15 permits unlicensed devices in the FM spectrum,
but not in the TV spectrum.
John Willkie
_____
De: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] En
nombre de Richard Hollandsworth
Enviado el: Thursday, August 09, 2007 2:11 PM
Para: opendtv@xxxxxxxxxxxxx
Asunto: [opendtv] Re: RV: Re: Charles Rhodes on unlicensed devices and white
spaces
If you punch in your zipcode (or better yet your street address)
to www.antennaweb.org it will give a list, sorted by predicted
signal strength, for nearby NTSC/ATSC stations.
The candidate "White Spaces" would be those NOT on the list.
However, antennaweb.org severely underestimates which stations
are receivable (countered by entering a much higher antenna height)
and does a miserable job of finding Low Power/Repeater
stations located in your back yard.
www.tvfool.com does a much better job of finding ALL stations,
listed in predicted signal strength order....pick the channels
NOT on the list...or at least the ones with weakest signal.
As part of White Space Device initialization, the user could enter
their zipcode/zipplus/street address/Lat-Lon/whatever and the WSD
on-line set-up routine could provide an ordered list of candidate freqs.
Both of these programs are based and periodically update from
the FCC TVQ database....which might not have the EXACT
EIRP of the day....but is usually very close, subject to the accuracy
of the data input by the affected TV Stations....giving them an
incentive to keep it accurate....
Of course, this only addresses how to find the "unoccupied"
DTV channels and doesn't address WSD vs WSD freq selection....
which is a WSD manufacturer design problem....
holl_ands
===================================
johnwillkie <johnwillkie@xxxxxxxxxxxxx> wrote:
Richard;
Your post verges on non-sensical.
Zip code areas are sometimes large. Would this database use the strongest
figure for the zip code, or the weakest? How will a zip code search tell
you if the station is operating at half-power for a few seonds, or greater
than authorized power?
Also, just how does a zip code tell one the power level varying over time?
I know you know of F(50,50) (service) and F(50,10) (interference)
probability. How would a zip code search tell you if you are in ,50 or ,10
conditions?
Having only spent an hour or two looking into this matter, I can tell you
that they are concentrating on detecting on immediate signal levels because
WITHOUT THAT, THEY DON'T have a snowball's chance in hell of getting the
thing through; the proponents have tried ALL OTHER KNOWN methods, and they
don't work (including geocoding more detailed than zip codes.). They
already tried the "no detection is needed" route, and that failed. If
you've got a better idea, you can probably make some money with it right now
.
As to cable, you must have missed something in your travels through FCC
documents. A BEDROCK principle of CATV in the U.S. is that cable owns their
ether and can set it up the way they want, largely. CATV is responsible for
all interference picked up and relayed by their plant, and cable cannot leak
their signal very much (particularly at aero frequencies) without imposing
big fines.
CATV is not responsible for interference within TV tuners (save for the
tuners within their STB's).
EMI for cable is a system issue; for tuners in general it's a part 15 type
certification issue, with the testing results on file. Cable wanted their
own environment, and now it's beginning to bite them.
I guess that's -1 for 2.
John Willkie
_____
De: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] En
nombre de Richard Hollandsworth
Enviado el: Wednesday, August 08, 2007 5:22 PM
Para: opendtv@xxxxxxxxxxxxx
Asunto: [opendtv] Re: Charles Rhodes on unlicensed devices and white spaces
Too much attention given to auto detect of "unoccupied" "White Spaces".
As if some "improved" sensitivity and/or algorithm will suddenly "fix"
everything.
[Why not simply enter zipcode to determine DTV assignments????]
Obviously, an indoor device with a negative gain antenna
(perhaps in ground floor appt...or suburban basement)
can't "detect" ALLLL occupied channels....
ATSC coverage from multiple directions is way too spotty....
But this little hiccup obfuscates the REAL problems.
DUE TO INTERFERENCE TO CABLE SYSTEMS, THERE ARE NO WHITE SPACES!!!!!
FCC OET tests determined that unlicensed devices would need
to be limited to 4 mW EIRP (vice desired 1 watt) based on
measurements at 2 meters separation....but this isn't "worst case".....
If a user fired up his brand new "White Space Device" on the other
side of a wall from his (or his neighbor's) DTV, the separation
could be only about a foot...reducing the allowable EIRP by 10+ dB.
[And many appt's don't have that 2-inch thick gypsum firewall.]
But the "EMI Susceptibility" test was conducted on only one
cable channel....if you've ever seen susceptibility test results,
you would know that some frequencies are more susceptible
than others....so there may be another 10+ dB reduction
whenever someone conducts tests across the entire VHF/UHF band....
But they only tested those (flat panel) DTV's that were "easy"
to transport to the test location....RPTV's may have less metal
shielding....and what about all of those people using USB Stick
Tuners connected to cable...where are their EMI test results???
Other than allocating these (unlicensed) devices to some other band,
or mandating that cable systems free up a "reserved" band (fat chance),
what other "options" are there that might be forthcoming
to somehow blow new life into this (hopefully) dying bird???
holl_ands
====================================
"Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx> wrote:
Craig Birkmaier wrote:
> A generic question here. Is a multi-carrier modulation system
> susceptible to the same interference issues, or does the use
> of multiple carriers help to mitigate the interference
> issues?
Same issues. Although I have read something written by Charles Rhodes
that stated, still without explanation though, that zero IF tuners work
better with multicarrier schemes than they do with single-carrier. I've
been waiting for the followup article on that.
There have been some excellent posts on this topic a few weeks ago. The
best and also cost effective solution today seems to be a tuned RF stage
followed by single conversion IF with active image cancelling circuit.
The tuned RF stage followed by dual conversion IF, which seemed to me to
cover all bases, just can't seem to match the cost of the single
conversion designs. And then there is the zero IF option, which I need
to find out more about.
>> One interesting stat he gives is that OTA signal strengths
>> under -68 dBm (what the FCC calls weak) occur in 84 percent of
>> DTT coverage area. (Note by area, not population.)
>
> Why should this be any different than it has been for NTSC?
> Obviiously the big sticks can't cover everything. This is the
> primary reason that there are thousands of TV translators
> around the country, trying to extend coverage to remote areas.
True. But even more obviously, SFNs can't cover everything. They improve
the immediate area over which they are deployed to the detriment of
outlying areas. The important point is that a -58 dBm signal is
considered close to the minimum margin for cable systems, yet 10 dB
*less* than that is very prevalent for OTA systems.
> There is also the issue of where the white spaces spectrum is
> in the highest demand. In general this will NOT be the remote
> areas, but rather, the urban areas where DTV signal strength
> should be relatively high. There is considerable interest in
> using the white spaces for rural broadband services, but this
> may be accommodated using properly designed services on
> channels that are adequately separated to prevent interference.
I think the auto-detection feature of these white space devices will be
their downfall. No matter where they are used, if they are deliberately
tuned for that location, they could be made to work.
The good news, though, is that these devices also interfere with cable
systems, and cable systems have no white spaces. So that should make
just about everyone concerned. Hopefully, this concern will prevent
white space devices from being prematurely marketed and then becoming a
royal pain in the behind for all of us.
Bert
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